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Seafloor Geomorphic Manifestations of Gas Venting and Shallow

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Seafloor Geomorphic Manifestations of Gas Venting and Shallow Distinctive rough seafl oor topography Seafl oor geomorphic manifestations of gas venting and shallow subbottom gas hydrate occurrences C.K. Paull1, D.W. Caress1, H. Thomas1, E. Lundsten1, K. Anderson1, R. Gwiazda1, M. Riedel2, M. McGann3, and J.C. Herguera4 1Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-9644, USA 2Natural Resources Canada, Geological Survey of Canada - Pacifi c, 9860 West Saanich Road, Sidney, British Columbia V8L4B2, Canada 3U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025, USA 4Centro de Investigacion Cientifi ca y de Education Superior de Ensenada, Carretera Ensenada-Tijuana No. 3918, Zona Playitas, C.P. 22860, Ensenada, B.C., Mexico ABSTRACT of slow seepage and shows the impact of gas et al., 2008; Jones, et al., 2010). These areas venting and gas hydrate development on the became targets for human occupied vehicle High-resolution multibeam bathymetry seafl oor morphology. (HOV) and remotely operated vehicle (ROV) data collected with an autonomous under- dives to conduct detailed observations and sam- water vehicle (AUV) complemented by com- INTRODUCTION pling programs. Water-column acoustic anoma- pressed high-intensity radar pulse (Chirp) lies have helped to identify other sites (e.g., profi les and remotely operated vehicle (ROV) Here, we report on seafl oor morphologies Merewether et al., 1985; Gardner et al., 2009). observations and sediment sampling reveal a observed where fl uids are venting along the The best grid resolution of ship-mounted distinctive rough topography associated with Pacifi c margin of North America. Areas where multibeam bathymetric data is ~10 m (e.g., seafl oor gas venting and/or near-subsurface hydrocarbon-bearing fl uids are seeping onto Hughes Clarke et al., 1998). While this level of gas hydrate accumulations. The surveys pro- the seafl oor are among the most dynamic deep- resolution nicely reveals the general shape of the vide 1 m bathymetric grids of deep-water gas sea environments. Chemosynthetic biological seafl oor, an appreciable gap exists between the venting sites along the best-known gas vent- communities surround these sites, supported morphologies that are resolvable in surface ship ing areas along the Pacifi c margin of North by energy from hydrocarbon oxidization (e.g., multibeam data and what can be observed using America, which is an unprecedented level of Sibuet and Olu, 1998; Levin, 2005). Diagenetic the fi eld of view provided by HOVs and ROVs. resolution. Patches of conspicuously rough reactions are enhanced in seep environments, Recently, it has become possible to use seafl oor that are tens of meters to hundreds notably those that result in the precipitation of autonomous underwater vehicles (AUV) to map of meters across and occur on larger sea- methane-derived carbonates (e.g., Ritger et al., selected areas of the seafl oor at 1 m grid resolu- fl oor topographic highs characterize seepage 1987; Kulm and Suess, 1990; Paull et al., 1992; tion. The AUV surveys presented here provide areas. Some patches are composed of mul- Bohrmann et al., 1998; Peckmann et al., 2001). a resolution that bridges the gap between visual tiple depressions that range from 1 to 100 m Gas hydrate formation and decomposition occur observations and the best resolution obtain- in diameter and are commonly up to 10 m within the near-seafl oor sediments, altering sea- able from surface ship mapping (≥10 m grids). deeper than the adjacent seafl oor. Elevated fl oor sediment properties (Kvenvolden, 1999; Because these surveys were collected with the mounds with relief of >10 m and fractured Sloan and Koh, 2008). As a consequence, sea- same vehicle, meaningful comparisons can be surfaces suggest that seafl oor expansion also fl oor seepage areas have become a focus of the made between sites, and the recurring charac- occurs. Ground truth observations show research community. Seafl oor seepage sites also teristics of seafl oor fl uid venting sites can be that these areas contain broken pavements pose special geohazard issues, in part because of delineated for the fi rst time. of methane-derived authigenic carbonates the potential for unstable seafl oor conditions and with intervening topographic lows. Pat- the possible existence of overpressured gas in METHODS terns seen in Chirp profi les, ROV observa- the near subsurface (Chiocci et al., 2011). Thus, tions, and core data suggest that the rough the hydrocarbon industry avoids installing sea- AUV Surveys topography is produced by a combination fl oor structures near seeps (Hough et al., 2011). of diagenetic alteration, focused erosion, and Most of the known seafl oor seepage sites Surveys were conducted at seven areas infl ation of the seafl oor. This characteristic were initially identifi ed by regional side-scan (Fig. 1; Table 1) where gas venting occurs texture allows previously unknown gas vent- sonar and surface vessel multibeam bathymetry using an AUV developed at the Monterey Bay ing areas to be identifi ed within these sur- surveys, because exposures of methane-derived Aquarium Research Institute (MBARI) specifi - veys. A conceptual model for the evolution of carbonates and chemosynthetic biological com- cally for seafl oor mapping (Caress et al., 2008). these features suggests that these morpholo- munities result in high seafl oor refl ectivity (e.g., The vehicle carried a Reson 7100, 200 kHz gies develop slowly over protracted periods Carson et al., 1994; Suess et al., 2001; Naudts multibeam sonar, and an Edgetech 2 to 16 kHz Geosphere; April 2015; v. 11; no. 2; p. 491–513; doi:10.1130/GES01012.1; 16 fi gures; 3 tables. Received 19 December 2013 ♦ Revision received 10 October 2014 ♦ Accepted 25 January 2015 ♦ Published online 27 February 2015 For permissionGeosphere, to copy, contact April [email protected] 2015 491 © 2015 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/2/491/3337001/491.pdf by guest on 25 September 2021 Paull et al. ples obtained in these dives and from the Inter- Neptune BC CA national Ocean Discovery Program (IODP) core A Barkley Canyon repository (Table 2) were processed in a simi- lar fashion. WA To determine sediment ages, ~25 cm3 sedi- ment samples from selected vibracores were B wet-sieved through a 63 µm screen. Tests of ~1000 planktonic (mixed species) or benthic Hydrate Ridge foraminifera were handpicked from the >63 μm Mexico OR NE Guaymas fraction. Where possible, two samples from vibracores were measured to constrain sediment SW Guaymas accumulation rates. Radiocarbon measurements were made at National Ocean Sciences Accel- erator Mass Spectrometry (NOSAMS) facility Eel Slump CA at the Woods Hole Oceanographic Institution (Table 2). A B RESULTS Figure 1. (A–B) Maps showing locations (red squares) where detailed surveys conducted with Monterey Bay Aquarium Research Institute’s (MBARI) mapping autonomous under- The seafl oor morphology of areas with fl uid water vehicle (AUV) reveal a distinctive recurring morphology (Neptune, Barkley Canyon, seepage is presented here as bathymetric grids Hydrate Ridge, and Eel Slump in A; and Guaymas Basin in B). Inset map (upper right) of 1 m resolution. An overview of previously shows location of parts A and B. All these areas are known to be associated with seafl oor published evidence and/or new ROV observa- fl uid seepage, and the pressure and temperature conditions at the seafl oor are within the tions documenting the occurrence of gas vent- gas hydrate phase stability zone. Location of previously published image from the Santa ing in the survey areas is also presented. Sites Monica Basin shown in Figure 16B is indicated with a triangle. CA—California; OR— are presented from north to south (Fig. 1). Oregon; WA—Washington; BC—British Columbia. Neptune Transect Chirp subbottom profi ler until the end of 2011, column with only INS navigation available until The existence of gas hydrate along the Casca- when it was replaced with an Edgetech 1 to 6 the seafl oor was within ~100 m. Data processing dia margin is well documented (e.g., Hyndman kHz Chirp subbottom profi ler. The AUV was was done using MB-system (Caress and and Davis, 1992; Westbrook et al., 1994; Hynd- preprogrammed to proceed to >200 waypoints Chayes, 1996; Caress et al., 2008). Subbottom man et al., 2001; Novosel et al., 2005; Riedel during each dive. Missions lasted up to 18 h depths to refl ectors imaged in the Chirp profi les et al., 2002, 2005, 2006a). Three contiguous and were designed for the vehicle to run at a are reported in meters below seafl oor (mbsf) AUV dives mapped the surface of an elongate speed of 3 knots while maintaining an altitude assuming a sound velocity of 750 m per second plateau in the vicinity of the Bullseye Vent node of 50 m off the seafl oor. Track lines were spaced two-way traveltime. on the Neptune cable (Fig. 2). Five subareas of ~150 m apart. In this mode, the AUV obtains distinctive rough topography identifi ed within overlapping multibeam bathymetric coverage at ROV Observations, Sampling, and these three dives are discussed here: Bullseye a vertical resolution of 0.15 m and a horizontal Sample Processing Vent, Bubbly Gulch, Spinnaker Vent, and two footprint of 0.7 m, and Chirp seismic-refl ection ridge crests. profi les with a vertical resolution of 0.11 m. Ini- The ROV Doc Ricketts was used to explore tial navigation fi xes were obtained from global the seafl oor associated with distinctive seafl oor Bullseye Vent positioning system coordinates when the AUV morphologies identifi ed in the AUV surveys Bullseye Vent is associated with a distinct was at the sea surface, and subsequently updated (Fig. 1; Table 1). Doc Ricketts dives provided 305-m-long, 65–75-m-wide, NE-SW–oriented with a Kearfott inertial navigation system (INS) ground truth verifi cation through the collection depression, developed on the surface of a broad and a Doppler velocity log (DVL).
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